Color images formed by multiple diffraction gratings

ABSTRACT

Method and apparatus for full-color reproduction of a continuous tone color picture or scene using multiple diffraction gratings. The reproduction consists of a plurality of small diffraction gratings (70). A plurality of color separation masks (60) are created for each unique scene. Those picture elements in the original scene containing a given primary color are captured in these transmission-type masks. In certain zones dictated by the separation masks, a plurality of interference patterns are recorded in the properly photosensitized media (70). The spatial freqency of said interference patterns correlates to the primary color to be reproduced. These interference patterns become multiple diffraction gratings when properly developed. The properly photosensitized media may be used as a master for replication purposes. When either master or replica (78) are properly illuminated in white light (76) and properly viewed, the multiple diffraction gratings act to reproduce the colors in the original scene.

This invention relates to full color reproduction of continuous tonecolor pictures or images by the use of diffraction techniques. Thereproduction, i.e., the device, consists of a regular array of smallplanar diffraction gratings of multiple spatial frequencies. Theconfiguration of these arrays is analogous to arrays of color halftonedots. The full color image so created is viewed in white light.

BACKGROUND OF THE INVENTION

In the conventional color halftone, as, for example, in a colortelevision picture, a continuous family of colors is simulated by thesuperposition or complementary juxtaposition of three "single color"halftones or "separations". These three colors are often referred to asthe three "primaries". For example, there may be a red separation, ablue separation, and a green separation.

In magazine or color print lithography, different primary color dots aredeposited on the paper or other medium in various sizes or diameters sothat the human eye integrates these dots and interprets them as variouscolors depending upon the dot mix. In photography, color negatives areprovided which detect the various colors passing through the lens in areversal process and, when printed, the various colors become viewableby the human eye. In all of these instances, however, the colorperceived is due to either the reflective, transmissive, absorptiveand/or radiant characteristics of the media (e.g., toners, inks, dyes,phosphors, pigments, etc.) involved--not the dispersive or diffractiveproperties.

According to the present invention, a full color image is created and isviewable in white light without the aid of colored inks, toners,sources, guns, or phosphors. A full color reproduction of a colorpicture, image or scene is realized by means of diffraction techniques.Instead of small, colored dots ("colored" by virtue of their reflective,absorptive, or radiant qualities) the halftone dots here produce theircolor by means of diffraction. The "dots" in this case are small"microgratings" whose grating spatial frequency has been chosen todiffract a particular portion of the visible spectrum in the directionof the viewer. Using additive primary methods, synthesis of anyarbitrary color, (not necessarily a pure spectral color) is possiblewhen the spectra from a plurality of different diffraction gratings arecombined in the same plane.

The term "primary grating" refers to grating areas with the same spatialfrequency. These "primary gratings" reproduce the red, green, and blueportions of the original image, and may be spatially combined in anumber of ways. For example, analogous to color television techniques, athird of the area can be apportioned for the red dots; a third for thegreen, and a third for the blue. The dots/screens do not overlap. Thiswill be referred to as a "complementary" screen design--one requiring aspecially designed halftoning contact screen.

As in conventional color lithographic processes, however, here too itmay be possible to allow the gratings to overlap or superimpose just ashalftone dots overlap. This approach obviates the need for a specialhalftoning contact screen--and thus permits the use of conventionalcolor separation methods. Again, just as in conventional color halftoneprinting methods, dot size will determine the net tone value of thatparticular halftone cell or dot.

DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference may be hadto the following detailed description of the invention in conjunctionwith the drawings wherein:

FIG. 1 shows the dispersion characteristic of three coplanar diffractiongratings of the present invention;

FIG. 2 is a schematic diagram of the present invention showing thecreation of the separation masks;

FIGS. 3A to 3C are diagrams showing complementary "red", "green", and"blue" halftone contact screens;

FIG. 4 is a schematic diagram of the present invention utilizing aLloyd's mirror arrangement to record the multiple diffraction gratings;

FIGS. 5A and 5B are a schematic diagrams of the recording of thedifferent grating frequencies utilizing the Lloyd's mirror arrangement;

FIGS. 6A to 6C are representative curves showing the various spectralcharacteristics of the illumination system, color separation filters anddiffraction gratings; and

FIG. 7 is a schematic representation of a person viewing a multiplediffraction grating prepared in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In this invention, the individual halftone patterns consist of a uniquevariety of "halftone dot" in which the variously sized dots are notcolored per se, but are small diffraction gratings. The spatialfrequency of a given diffraction grating is chosen to diffract aparticular wavelength into a particular angle. The other screens wouldconsist of dots having different diffraction grating frequenciesdesigned to diffract other wavelengths into the same viewing angle.

FIG. 1 of the present application discloses one variation on themultiple diffraction grating technique of the present invention. F1, F2,and F3 are different reflective diffraction gratings all in the sameplane, but not necessarily superimposed over one another.

White light illumination is incident on this device near grazingincidence; i.e, at 85 degrees from the grating surface normal. One thenfinds three characteristic spectra associated with each of the threegrating frequencies. The visible blue-to-red portions of the first-orderspectra are shown for each grating in the figure. The gratings have beendesigned in such a way that at a 20 degree viewing angle, the (red) 630nm light from F1 coincides with both the (green) 530 nm light from F2and the (blue) 470 nm light from F3.

Table 1 below describes a particular combination of three gratingsdesigned for illumination at 85 degrees and viewing at 20 degrees. 630nm light is chosen for red, 530 nm light for green and 470 nm light forthe blue.

Table 2 below describes a particular combination of three gratingsdesigned for illumination at 85 degrees and viewing at 0 degrees. (Itshould be noted that the first order diffracted light due to a beamincident at plus 85 degrees and the minus first-order diffracted lightdue to a beam incident at minus 85 degrees both coincide at zerodegrees--the design viewing angle. This symmetrical design enables oneto increase the relative illumination incident on the gratings.)

                  TABLE 1                                                         ______________________________________                                        illumination at +80 degrees;                                                  nominal viewing angle = +20 degrees                                           DIFFRACTION ANGLE, degrees                                                           RED    GREEN    BLUE                                                          630 nm 530 nm   470 nm   DISPERSION                                    GRATING FREQUENCY l/mm                                                        ______________________________________                                        F1 = 2120                                                                              20       7.5      0.1    20                                          F2 = 2520                                                                              36.5     20       11.0   26                                          F3 = 2850                                                                              52.9     30.9     20     33                                          ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        illumination at plus or minus 80 degrees;                                     nominal viewing angle = 0 degrees                                             DIFFRACTION ANGLE, degrees                                                           RED    GREEN    BLUE                                                          630 nm 535 nm   470 nm   DISPERSION                                    GRATING FREQUENCY, l/mm                                                       ______________________________________                                        F1 = 1563                                                                              0°                                                                              -8.5°                                                                           -14.5°                                                                        14.5°                                F2 = 1841                                                                              10.1°                                                                           0°                                                                              -6.9°                                                                         17.0°                                F3 = 2095                                                                              19.6°                                                                           7.8°                                                                            0°                                                                            19.6°                                ______________________________________                                    

In the example described above in Table 1, an observer looking at thesethree gratings at an angle of 20 degrees from the grating normal will,with 85 degree illumination, see a new color which is the combination ofthe "red" portion of the incident spectrum dispersed by frequency F1grating areas, the "green" portion of the spectrum dispersed byfrequency F2 grating areas, and the "blue" portion of the incidentspectrum dispersed by frequency F3 grating areas. The actual colorperceived due to any single grating alone will be dependent upon itssize, and shape, its grating spatial frequency, its diffractionefficiency and the viewing and illumination conditions. Reviewing theparameters which will influence the appearance of this device are:

the particular observer,

grating frequency (lines per millimeter),

grating size and shape,

grating diffraction efficiency,

viewing angle,

viewer distance to device,

illumination spectral content,

illumination angle of incidence,

illumination source size.

In the present embodiment, as in conventional halftone techniques, dotarea determines the "quantity of color" that will be viewable by anobserver. By virtue of the relatively small size of these "diffractiondots" and the close proximity of dots of different spatial frequencies,a new color will be perceived from this combination of three relativelypure spectral colors.

A master multiple diffraction grating color reproduction can begenerated by at least three different techniques: First, a computergenerated technique in which the separation masks are synthesizeddigitally and which may include conventional electronic color separationmethods; second, in which the separation masks are created using aspecially designed halftone contact screen; and third, similar to thesecond except for the use of a standard halftone contact screen. Thesecond technique is described in detail below.

(In the third process method, the grating exposures are allowed tooverlap, as in conventional color halftoning techniques. In so doing, itis expected that diffraction efficiency of any given grating will becompromised as double and triple exposures in the same place on themaster plate would be likely. Such an approach, however, lends itself tothe use of existing halftoning materials and techniques; that is, nospecially designed halftone contact screens would be required.)

A. Create separation masks for each unique color original

The process begins, for example, with a continuous tone colorphotograph. Since it is attempted to form a colorimetrically faithfulreproduction of this continuous tone color photograph, needed to beknown are the component colors with which the multiple diffractiongrating color image will be "played back". If the reproduction will beplayed back using three diffraction gratings which, for example,diffract red light centered at wavelength W1, green light centered atwavelength W2, and blue light centered at wavelength W3, all into aparticular viewing angle, only then can the separation masks be properlycreated.

The separation masks, which will be used in the recording of themultiple diffraction grating master, can be created in a manner similarto the way conventional color halftone screen separations are nowcreated. The major components that are seen in FIG. 2 are:

The original color picture 10;

White light illumination 20;

Conventional process camera with registration means for both object,contact screen, and film. 30;

Specially designed color separation filter 40;

Specially designed transmission type contact halftone screen 50;

High contrast panchromatic lithographic film 60.

The original color picture 10 is placed in front of the process camera30 which forms a real image on the lithographic film 60, upon beingilluminated by white illumination source 20.

The specially designed, transmission-type color separation filter 40 isplaced in front of the process camera lens. Referring to FIG. 6, thecolor separation filters must be designed with due consideration to thespectra of the primary grating that will be used in playback. To firstorder, the transmission spectrum of the color filter must match thediffraction efficiency spectrum of the particular grating.

As schematically shown in FIG. 6, the spectral transmissioncharacteristic of these filters will be functionally similar to thediffraction efficiency curves of the primary gratings.

Assuming both a positive master and a positive lithographic film 60,filter 40 must transmit the particular spectral color to be renderedwhen this separation is played back.

The specially designed transmission-type contact halftone screen 50 isplaced directly in registration and in contact with the lithographicfilm 60 to be exposed. That portion of the white light illumination 20spectrum, both reflected by the object and transmitted by the filter,will result in the corresponding areas on the positive lithographic filmto become clear after passing through the halftone screen 5.

FIGS. 3A, 3B, and 3C schematically show three possible "complementary"halftone contact screens. ("Complementary" here refers to the fact thatone-third of the final image area is dedicated to separating out the redprimary content in the original, one-third of the final image isdedicated to separating out the green primary content in the original,and one-third of the final image is dedicated to separating out the blueprimary content in the original.)

It is noted that in all of these designs, at least two thirds of thearea of the mask is opaque so as not to expose areas apportioned for theother screens; the remaining one third area consists of relatively clearzones with a sinusoidal transmission cross section. Diamond zone shapesare shown in FIGS. 3A-3C; while other geometries are, however, possibleas in conventional halftoning, the resulting clear lithographic dot sizewill be a direct function of the exposure. It is noted that when any twoscreens are superimposed in registration there is no transmission.

Assuming a positive working lithographic film, the clear zones in eachprimary separation mask will correspond to only those zones in theoriginal with spectral content in the transmission band of the filter;the size of these zones will be dictated by the following product:

    DOT SIZE=INTEN.sub.source (dw)×R.sub.object (dw)×T.sub.filter (dw)×T.sub.screen (dw)×SENS(dw),

where: ##EQU1## "dw" is the narrow spectral band over which all theabove parameters have relatively constant values.

The second and third separation masks are formed similarly, each willuse its own color filter and either a new contact halftone screen, orthe same screen with a different angular orientation.

We now have a complete set of three separation masks 60 for thereproduction of the original color picture, image, or scene.

B. Using interferometric techniques, record gratings on a single masterin transmission zones of separation masks

On a single lithographic plate then, FIG. 4, with the appropriatephotosensitive coating, an interferometric diffraction grating exposureis made through each of the lithographic type transmission separationmasks described above. A "Lloyd's mirror" arrangement 62 in conjunctionwith a vacuum platen, not shown, as shown in FIG. 4 can be used toeasily "impress" a different diffraction grating through each of thethree masks. The masks are used in registered contact with thephotosensitive coating on plate 70 and thus act as "stencils", allowingthe grating to be recorded only in the areas corresponding to the clearportions of the mask. In this example, three masks would require threeseparate, registered, exposures.

The Lloyd's mirror configuration produces an interference patternbetween two monochromatic beams that are derived from the samesource/laser 48. Linear interference fringes (bands of light and dark)are thus recorded in the positive working photosensitive material 70which is placed in the interference zone as shown in FIGS. 5A and 5B. Atthis point the gratings-to-be may be thought of as being latent imagesof the interference patterns. The single master plate will undergo threeserial exposures--each interferometric exposure with a particularseparation mask and a particular grating frequency.

C. Develop master plate

Utilizing positive photoresist as the photosensitive media, andfollowing proper development, the grating latent images are transformedinto surface relief patterns which function as diffraction gratings ofthree discrete spatial frequencies.

The exposure, development, and replication processes will be optimizedwith respect to grating diffraction efficiency and with dueconsideration to the following parameters:

grating type: either reflection or transmission,

viewing and illumination angles,

wavelength of interest.

D. View master under proper illumination and viewing conditions

Under white light illumination 76, the multiple diffraction gratingreproduction 78 will be visible in reflection even in the absence of areflective coating. In FIG. 7, a linear, white light line source ofillumination is shown. A long tungsten filament lamp would be such asource. Cylindrical collimation optics 74 are schematically shown. Thesewould be designed to provide a collimated flood of white light onto thegrating halftone at a single narrow angle of incidence. In FIG. 7, theangle of incidence shown is approximately 85 degrees as calculated inthe above examples.

The photoresist on glass device created above then becomes the masterfrom which conventional electroforming techniques can be applied to massproduce full-color replicas of the original color image or scene. Theabove process is repeated for each new color original or scene.

E. Electroform embossing tool from above master

Conventional nickel electroforming and embossing-on-aluminized-mylartechniques may now be applied to make multiple replicas from the abovemaster. These are the same techniques currently being used to makereflective hologram replicas.

These flexible sheets of embossed aluminized gratings, as before, arebest viewed under the design illumination and viewing conditions.

While the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made without departingfrom the essential teachings of the invention.

What is claimed is:
 1. A method of full color reproduction of continuoustone color pictures, images or scenes by the use of diffractiontechniques comprising:illuminating (20) a color picture or scene (10) tobe reproduced, separating (40) the reflected light from said picture orscene into its primary component colors, imaging (30) said separatedcomponent colors one at a time onto and thereby exposing a separatephotosensitive film (60) for each component color, previously coveringeach of said photosensitive films with an associated individual halftonecontact screen (50), one for each component color, each of said filmscomprising individual separation masks, one for each component colorcorresponding to the primary component colors reflected from saidpicture or scene, exposing (70) a photosensitive plate with acharacteristic interference pattern for each primary component colorthrough said individual separation masks to form latent multiplediffraction gratings on said photosensitive plate representative of theprimary colors reflected from said picture, image or scene, anddeveloping said plate so as to convert the latent interference patternsinto surface relief patterns with multiple spatial frequencies whichexhibit characteristic diffraction effects and thereby reproduce theoriginal color image, picture or scene.
 2. The method as set forth inclaim 1 further comprising illuminating (76) said photosensitive platewith white light so as to reveal said color image by diffracted light.3. The method as set forth in claim 2 further comprising placing saidwhite light illumination source at a predetermined angle to saidphotosensitive plate, and viewing said light at a second predeterminedangle so as to reveal the true colors in the original picture, image orscene.
 4. The method as set forth in claim 1 further includingreplicating said multiple diffraction grating by using saidphotosensitive plate as a master of said picture, image or scene.
 5. Amultiple diffraction grating in the form of a plurality of halftoneareas on a substrate (70), each of said halftone areas having at leastone of a plurality of diffraction grating frequencies, such that theprimary component colors in an original picture, image, or scene arereproduced when properly illuminated and viewed, wherein said pluralityof grating areas comprises a first pattern of diffraction grating areaswith a first grating spatial frequency representative of a first primarycolor, a second pattern of diffraction grating areas with a secondgrating spatial frequency representative of a second primary color, anda third pattern of diffraction grating areas with a third gratingspatial frequency representative of a third primary color.
 6. Themultiple diffraction grating as set forth in claim 5, wherein saidsubstrate comprises a photosensitive plate upon which the plurality ofinterference patterns have been recorded and developed to reveal saiddiffraction gratings.
 7. The multiple diffraction grating as set forthin claim 5 wherein said substrate serves as a master for the purpose ofmaking replicas of said multiple diffraction grating.
 8. Apparatus formaking a full color reproduction of continuous tone color pictures,images or scenes by the use of diffraction techniques comprising:meansfor illuminating (20) a color picture or scene (10) to be reproduced,means for separating (40) the reflected light from said picture or sceneinto its primary component colors, means for imaging (30) said separatedcomponent colors one at a time onto and thereby exposing a separatephotosensitive film (60) for each component color, means for previouslycovering each of said photosensitive films with an associated individualhalftone contact screen (50), one for each component color, each of saidfilms comprising individual separation masks, one for each componentcolor corresponding to the primary component colors reflected from saidpicture or scene, means for exposing a photosensitive plate (70) with acharacteristic interference pattern for each component color throughsaid individual separation masks to form latent diffraction gratings onsaid photosensitive plate representative of the colors reflected fromsaid picture, image or scene, and means for developing said plate so asto convert the latent interference patterns into surface relief patternswith multiple spatial frequencies which exhibit characteristicdiffraction effects and thereby reproduce the original color image,picture or scene.
 9. The apparatus as set forth in claim 8 furtherincluding means for illuminating said photosensitive plate means withwhite light (76) so as to reveal said color image by diffraction lightreproduction.
 10. The apparatus as set forth in claim 9, wherein saidilluminating means is placed at a predetermined angle to saidphotosensitive plate means such that the true component colors in theoriginal scene or picture will be reproduced when viewed at a secondpredetermined angle.
 11. The apparatus as set forth in claim 8 furtherincluding means for replicating said multiple diffraction grating byusing said photosensitive plate as a reproduction master of saidpicture, image or scene.
 12. The multiple diffraction grating as setforth in claim 5, wherein said plurality of grating areas exhibitcharacteristic diffraction effects such that said multiple diffractiongrating is a viewable representation of said original color picture,image, or scene.